I bought a copy of this book last summer and found it immensely helpful in understanding some of the actions of the priest and deacon at our divine services. It is particularly helpful to me as somebody who, while knowing the general roles, does not have much experience of services at which a deacon is present and serving. Despite the fact that our services rely heavily on the deacon and can seldom be offered fully without one, the reality is that most Orthodox people - clergy and laity alike - are unaccustomed to having a deacon present, and when one is fulfilling his duty, it shows: priests who are accustomed to doing everything themselves forget where the end of their role lies and where that of the deacon begins (or perhaps, due to having served only a short diaconate, they never learnt in the first place), choirs forget (or are possibly unaware of) the different responses and methods that are employed when the Liturgy is served with a deacon, and so forth.
Having said that, this work is not without its critics, and there are those clergy who are knowledgeable about Russian liturgics who claim that the book bears some inaccuracies. I am no expert so can only speculate as to the nature of these disagreements, although I, myself, have spotted two points on which my experience and reading differ from Fr Gregory's direction. They may simply reflect regional variations in practice (Russia is not a tiny place, and the young church of the diaspora was made up of people from many areas who took different customs with them).
Of course, the differences may also be due to the author's past as a Uniate priest. Readers may be aware that, after many centuries of latinisation, the eastern-rite churches subject to Rome have been encouraged by two successive popes (Messrs Wojtyla and Ratzinger) to return to their eastern roots, both in terms of liturgics and devotional culture. In some churches, such as the Ukrainian Greek Catholic church, of which Fr Gregory was a priest before becoming Orthodox, this has taken the form of a return in many areas to the precise letter of the typikon, even on points where living Orthodox custom has developed beyond that. Therefore, in videos from the fascinating church of St Elias in Canada, for instance, we witness practices that are only to be found in rubrics in most of the Orthodox Church: the two choirs joining and parting at specific times, deacons fanning the Gifts throughout the Anaphora, censings performed in a cross-wise fashion, and so forth. Perhaps Fr Gregory's notes reflect the typikon on other points that are less obviously no longer followed. Again, this is merely speculation on my part.
Whatever the differences may be, surely they can only be minor, and any points requiring clarification can be referred to the bishop for his direction. Whatever disagreements there may be, I am certain that the Church in the English-speaking world, where many of the traditional means of training the clergy are difficult to encounter, is better off with this book than without it, and that we ought to be grateful for Father Gregory's labour of love.
May his memory be eternal!